1. Field of the Invention
The invention relates to television receivers and, more particularly, the invention relates to a video amplifier for a television receiver.
In modern television receivers, a microprocessor provides command and control information through an I2C bus interface to provide various control functions. The I2C bus is coupled to a module (an IF integrated circuit) comprising a tuner, IF, and stereo decoder. To achieve the best picture and sound performance, the microprocessor may control 8 or more alignment functions and various switch functions through the I2C bus interface. These functions provide factory alignment of various signal characteristics such as video output amplitude and dc-level, RF AGC delay threshold, and the like. Such electronic alignment is performed to ensure that a consistent picture quality between televisions occurs in retail show rooms; to ensure that consistent picture equality results between inputs of a television with multiple tuners or multiple auxiliary inputs; and to maintain signal levels within dynamic range limitations of the receiver circuitry.
U.S. Pat. No. 4,442,458 discloses an amplifier for driving a cathode ray tube (CRT). This device includes a first amplifier having an input terminal for receiving a video signal and an output terminal connected to the control grid of the CRT. A second amplifier is responsive to the cathode current of the CRT and provides an output which is combined with the output of the first amplifier. The first amplifier has a wider bandwidth and lower dynamic range than corresponding prior art amplifiers. The second amplifier has a higher dynamic range than the output signal of the first amplifier. As the outputs of the first and second amplifiers are combined, the dynamic ranges are added together thereby providing a wider bandwidth while maintaining the dynamic range of the output signal energizing the control grid at a desired range.
U.S. Pat. No. 4,706,108 discloses an automatic system for controlling the color gain, hue and white balance of a television monitor. The system includes a signal processor, a color CRT a reference signal memory and a signal control circuit connected to the signal processor and reference signal memory. The signal control circuit adjust the operation of the signal processor based on a standard television signal and reference signal from memory.
EP 0 359 493 A discloses a double display mode for differential gain that displays mirror image differential gain signals. A microprocessor controlled variable is added to the differential gain signals to bring one of two corresponding opposite extreme amplitude points of the differential gain signals into registration so they overlay each other. The microprocessor controller variable is then adjusted until the opposite corresponding extreme amplitude points of the differential gain signals overlay each other. The difference in value of the variable is then converted into a percentage gain differential value for display alphanumerically without the need for display graticules.
U.S. Pat. No. 5,311,295 discloses an RGB display of a transcoded serial digital video signal. The display is produced by converting the serial digital video signal to a parallel digital video signal, separating the digital luminance component signal and the two digital difference component signals from the parallel digital video signal. The digital luminance and color difference component signals are then converted into analog encoded color component signals. The analog encoded color component signals are then transcoded into RGB component signals and the RGB component signals are routed to a waveform display device.
One aspect of electronic signal alignment control that is not presently addressed in the art is the ability to control the DC level of a video signal that is produced by an IF integrated circuit. Such a signal alignment feature enables the IF integrated circuit to be matched to down stream circuits such that the IF integrated circuit can be used in a greater number of television receivers.
Therefore, there is a need in the television receiver art for an economical solution that enables a system microprocessor to control the DC level of a video signal.
The present invention is a method and apparatus within a television receiver for electronically aligning signals within the receiver by controlling the DC level of a video signal. The television receiver comprises a IF integrated circuit that provides a tuner, IF and stereo decoder. The invention uses a digital-to-analog converter (DAC) integrated circuit to control a DC level produced by a video amplifier that is coupled to the output of the IF integrated circuit. In one embodiment of the invention, a system microprocessor is coupled through an I2C bus to a DAC that controls the video amplifier. The video amplifier comprises a DC video level circuit that applies a DC level to the amplified video signal. Additionally, the video amplifier is a non-inverting amplifier such that the output of the IF integrated circuit is polarity matched to downstream circuitry.